1. Field of the Invention
The invention relates to electronic weighing devices. More particularly, the invention relates to an electronic weighing device which employs surface acoustic waves to measure weight.
2. State of the Art
From very early times, civilizations used weighing machines in business transactions. The simplest form of a weighing machine is called the balance because it balances two loads from opposite ends of a horizontal bar. The Egyptians used balance instruments more than 4,000 years ago. They balanced a beam from a vertical support and suspended two pans from either end of the beam. One pan held a known weight, While the other held the object to be weighed. The delicate mechanical precision balances still used today in scientific laboratories are based on the same principles. The first true precision balance was probably built around 1770. The precision balance achieved its accuracy by using an adjustable center of gravity and a knife-sharp fulcrum. Modern precision balances, known as macrobalances, are sensitive enough to measure loads up to 200 grams to the nearest 0.1 milligram. Even more sensitive balances, called microbalances, can weigh a maximum load of 0.1 gram to the nearest 1 millionth of a gram. Most precision balances are kept in glassed-in cabinets so that they are unaffected by external temperature changes and drafts.
Balances require the use of a known, or standardized, weight which is at least equal to the heaviest load to be weighed. For weighing heavier loads, there are scales based on the "steelyard" which was first invented by the Romans. The steelyard has a suspended beam with two unequal arms or levers which allows the use a small weight to balance a heavy load. The load is hung from the short arm, near the fulcrum, and a weight, or counterpoise, is moved along the longer arm until a balance is reached. Markings on the long arm indicate the weight.
Spring scales were invented around 1700. Spring scales may be either compression-type or tension-type. In either case, the stretching or compressing of a spring moves a pointer to register the weight on a dial or moves a portion of a transducer to provide an electrical signal which is then used to register weight electronically.
Modern electronic scales measure loads faster than traditional scales or balances and electronic measurements are easily displayed, stored, and transmitted. Electronic scales use one of three principal mechanisms: the strain-gauge load cell, the pressductor, or the force balance. In load cell scales, the applied weight compresses a column which has a strain gauge bonded to its surface. The strain gauge is a fine wire which undergoes a change in electrical resistance when it is either stretched or compressed. A measurement of this change in resistance yields a measure of the applied weight.
A pressductor is a laminated magnetic block with four holes drilled through it. A wire carrying alternating current is threaded through two of the holes, and a pickup wire connected to a voltage meter is threaded through the other two holes. In the absence of a load, no voltage is detected by the pickup wire. However, an applied load induces a voltage that is proportional to the weight. Both the load cell and the pressductor are used to measure relatively large weights, e.g. from 1 kilogram to many tons.
The force balance utilizes a moving electromagnetic coil which is deflected by an applied weight. The deflection is corrected by a balancing current which is proportional to the deflecting weight. Force balance scales are used to measure relatively small weights.
Throughout history new and improved scales have been designed to achieve greater accuracy and to simplify their use. Currently, the most accurate scales are not simple to use and their range of accuracy is limited, i.e. they are only accurate for a certain range of weight. Moreover, accurate scales are relatively expensive.